With the high integration of a nonvolatile semiconductor memory device, a width of a floating gate electrode and a width of an isolation groove are reduced. Reduction in width of the isolation groove degrades burying characteristics of a control gate electrode material after forming an inter-electrode dielectric film.
Moreover, reduction in width of the floating gate electrode increases difficulty in processing and influence on variation in writing characteristics by a processing variation. If the inter-electrode dielectric film is made thinner and thereby improves a coupling property of the floating gate electrode and the control gate electrode to avoid such problem, a life of an element used in a low electric field of a peripheral circuit region deteriorates.